The present invention is directed to an electro-optical display device, particularly a liquid crystal display device, with a medium, which is switchable between optical different states, disposed between two carrier plates, which carrier plates on a surface facing the medium are provided with electrically conductive layers which are each covered by a dielectric layer.
Electro-optical displays utilizing a liquid crystal layer disposed in a cell formed by a pair of carrier plates having electrodes formed by transparent conductive coatings covered by dielectric coatings or layers are known in various types of embodiments. For example, German Offenlegungsschrift No. 2,458,883 and the corresponding British Patent Specification No. 1,481,719, which are both based on the same French priority document, disclose one type of display device.
As is well known, the light, which permeates the transparent layers, is partially reflected at the interface which have non-sequential layers in the index of refraction. Because of this, the electrode coatings of a display will also reflect the light, namely to such a degree that representation qualities would be significantly prejudiced without special dereflection or anti-reflection measures.
A conductive layer is then relatively low reflection when it has a thickness which corresponds to the half wavelength of the light in the conductive layer of material or when the thickness, which is measured with respect to the light wavelength, is relatively thin. The two dimensions have a series of specific disadvantages. .lambda./2 layers, which are, for example approximately 150 nm thick in an In.sub.2 O.sub.3 electrode, must be exactly dimensioned, are often no longer significantly anti-reflective ("color cast") at specific light wavelengths, and generally lead to orientation disruptions in liquid crystal cells at their edges. The "thin" conductive layers, which generally have a thickness of at most 25 nm, have a relatively high resistance and always still exhibit clear preceptible residual reflections especially when applied by a chemical vapor deposition (CVD) process.
To overcome these difficulties, one suggestion has been the insertion or the use of reflection reducing additional layers (see German Auslegeschrift No. 2,313,730 and the corresponding British Patent Specification No. 1,428,700 based thereon as well as the previously mentioned German Offenlegungsschrift No. 2,458,883). These layers, which are usually located between the substrate and the electrode layer, have a thickness of .lambda./4 and according to the disclosure of the German Offenlegungsschrift No. 2,458,883, exhibit an index of refraction ##EQU3## However, these layers do not provide completely satisfying results and the reflection factors will deviate significantly from one another between the area with the conductive layer and the area free of the conductive layer. The additional layers could be expanded to form "anti-reflection coatings" as disclosed in the U.S. Pat. No. 3,736,047 and the German Offenlegungsschrift No. 2,454,462. Many experts in the field, however, have arrived at the opinion that the anti-reflection or de-reflection coatings cannot justify the significant additional expense involved at least in a liquid crystal display (cf. in this regard M. Tobias, International Handbook of Liquid Crystal Displays, Ovum Press Limited, London, 1975, Section 8.6.5.1). Thus, praxis, one has the recourse to provide the electrodes with a more or less precisely defined thickness and accept the mirror effect.